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Old 15th September 2012, 09:56 PM   #1181
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I think that I am following the discussion of wiring the parallel capacitors here but could someone put up a line drawing of the wiring, not in schematic form, but a physical layout. It would be much appreciated. And would the last capacitor be a film cap?

Steven
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Old 15th September 2012, 10:11 PM   #1182
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Originally Posted by DF96 View Post
With careful wire/track positioning it might be possible to reduce the effective inductance of capacitors. Two caps in parallel have an average current path which runs between them, so if the wire runs here then the net inductance is minimised.
DF,

Note that I wasn't talking about paralleling caps onto a buss with only two conductors that all of the caps share. Each capacitor would have to have its own two conductors and all of the pairs of conductors would not be connected anywhere except at the decoupling point.

There is no "might" here, that I am aware of. Henry W. Ott describes this technique rather thoroughly, in the latest edition of his book, "Electromagnetic Compatibility Engineering". Also, see some of the material by Bruce Archambeault (sp?), on the web.

Tom
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Old 15th September 2012, 10:24 PM   #1183
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Quote:
Originally Posted by Kindhornman View Post
I think that I am following the discussion of wiring the parallel capacitors here but could someone put up a line drawing of the wiring, not in schematic form, but a physical layout. It would be much appreciated. And would the last capacitor be a film cap?

Steven
Argh!! Where does this idea of always throwing in a parallel film cap keep coming from?

Please read the thread about paralleling electrolytics and film caps.

It is NOT, repeat NOT, a good idea, especially way out at the PSU reservoir caps. It MIGHT, but only might, be safe-enough, and would even have a chance of being effective, right at the load, for decoupling.

Similar "discouraging" comments are waiting for the next mention of placing a film cap (or any cap, by itself) across a rectifier diode. <grin>

OK, with that all having been said, the paralleling of multiple capacitors could apply to electrolytics, but could also be applied with any type of capacitors. It is particularly important for decoupling in high-speed applications, when using ceramic or film caps. It can also be necessary for decoupling in audio power amplifiers, where inductance can be too high with only one cap.

I will do a drawing for you, in a few minutes.

Tom
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Old 15th September 2012, 10:27 PM   #1184
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Thank you Gootee, I did propose p (Power Supply Resevoir Size)

I am following your hunt for p
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Old 15th September 2012, 10:36 PM   #1185
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How to parallel capacitors to reduce overall inductance and resistance:
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Old 15th September 2012, 10:49 PM   #1186
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assume p
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Old 15th September 2012, 10:53 PM   #1187
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Thank you Gootee. So rather than daisy chain the capacitors we tie all the leads together as if it was a star ground connection. Do all the leads need to be the same length or does that not matter and should the wires all be parallel to each other before the connection?
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Old 15th September 2012, 10:58 PM   #1188
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practically it would look like this
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Old 15th September 2012, 11:25 PM   #1189
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assume p
No, sorry. When current is needed, the voltage across the common terminals will be lower than any of the individual cap voltages. They will all then supply current when needed, just with superior inductance and resistance parasitics compared to one cap.
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Old 15th September 2012, 11:33 PM   #1190
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Originally Posted by Kindhornman View Post
Thank you Gootee. So rather than daisy chain the capacitors we tie all the leads together as if it was a star ground connection. Do all the leads need to be the same length or does that not matter and should the wires all be parallel to each other before the connection?
Lengths could be somewhat different. That just changes the parasitics a little, and changes the caps' utilizations (providing current) slightly, unless the differences were large.

Usually, the connections should be as short as possible, since we're usually also worried about the parasitic inductance and resistance of the conductors.

By "parallel to each other", you would mean "not connected together until they reach the point where they join, at the last possible moment, as close as possible to where the capacitance is needed". Geometrically parallel or not would not usually be very relevant, unless maybe we were worried about tiny coupling effects. Electrically parallel is usually all that really matters, as far as "parallel" is concerned.
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